1. Field of the Invention
The present invention relates to a thyristor type semiconductor surge suppressor having an excellent breaking performance and an excellent surge operation performance.
2. Description of the Related Art
A bidirectional thyristor type surge suppressor having a basic structure, provided by P.sub.1 N.sub.1 PN.sub.2 P.sub.2 layers shown in FIG. 8A, and a current-voltage characteristic shown in FIG. 8B (showing only one direction) has been used as a surge suppressor for various electronic circuits because of its compact size and low cost.
This suppressor is used by connecting it between both terminals of an electronic circuit H to be protected as shown in FIG. 9. For example, when a surge voltage S having a voltage value exceeding V.sub.B0 of the surge suppressor Z in FIG. 8B is applied to a line of the electronic circuit H, the surge suppressor Z is immediately turned on to protect the electronic circuit H.
In this case, it it required that after the surge current flows through the surge suppressor Z, a current flowing through the electronic circuit by a power supply voltage E.sub.0 is immediately cut off to return the surge suppressor Z to a condition before the application of the surge voltage.
For this reason, as is well known, a holding current I.sub.H of the suppressor Z must satisfy the following relation: EQU I.sub.H &gt;E.sub.0 /R
where
R: circuit impedance PA1 E.sub.0 : power supply voltage
The holding current I.sub.H must be increased in order to obtain an excellent breaking performance.
As has been known, the holding current I.sub.H is determined by an impurity concentration and a thickness of respective layers of the thyristor type surge suppressor Z. However, the impurity concentration and the thickness cannot be easily controlled with high precision.
On the other hand, an increase in the holding current I.sub.H results in a decrease in a surge current capacity. Therefore, it is not always easy to increase the holding current I.sub.H and the surge current capacity at the same time. In addition, it is difficult to make the thyristor type surge suppressor since the impurity concentration must be precisely controlled.
In order to eliminate such problems, a thyristor type surge suppressor having a structure shown in FIGS. 10A and 10B has been proposed.
That is, an N-type base region N.sub.1 is provided on one surface of a P-type semiconductor substrate P, and an N-type base region N.sub.2 is formed on the other surface of the P-type semiconductor substrate P. A P-type emitter region P.sub.1 is formed in the base region N.sub.1 to expose a plurality of circular regions S.sub.1, and a P-type emitter region P.sub.2 is formed in the base region N.sub.2 to expose a plurality of circular regions S.sub.2. An electrode T.sub.1 is provided over the exposed base regions S.sub.1 and the emitter region P.sub.1, and an electrode T2 is provided over the exposed base regions S.sub.2 and the emitter region P.sub.2.
According to the semiconductor surge suppressor having the structure, as is apparent from the following operation, the holding current I.sub.H can be increased by the number of base regions S.sub.1 and S.sub.2 and their arrangement as compared with the conventional surge suppressor having no exposed base regions S.sub.1 and S.sub.2.
However, if the number of the exposed base regions and the area of each of the exposed base regions are increased to increase the holding current I.sub.H, a switching current I.sub.S shown in FIG. 8B is increased in proportion to the holding current I.sub.H. Therefore, even if the breaking performance can be improved, the operation performance against the surge is lowered.
First, considering that the semiconductor surge suppressor is turned on in a direction directed from the electrode T.sub.1 to the electrode T.sub.2. When an applied voltage exceeds a breakdown voltage V.sub.B0 of a junction J.sub.2, a current I consisting of current components I.sub.0, I.sub.1, and I.sub.2 flows through the surge suppressor as shown in FIG. 10B. When a junction J.sub.1 is forward-biased by a voltage drop that is given by the current component I.sub.1 and an effective lateral resistance R.sub.N of the base region N.sub.1, and the bias voltage exceeds the rise voltage of the junction J.sub.1, holes are then injected into the base region N.sub.1 from the Junction J.sub.1, so that a current flows between the electrodes T1 and T2. For this reason, the switching current I.sub.S is increased as compared with the structure having no exposed base regions S.sub.1.
Second, considering the turn-on state. The device does not conduct Just under the base regions S.sub.1, but conducts in only a portion Just under the emitter region P.sub.1. The turn-on state is kept by injection of holes from the junction J.sub.1 and injection of electrons from a junction J.sub.3. In addition, when the ON-sate is lowered to turn off the device, the holes injected from the junction J.sub.1 are recombined in the exposed base regions S.sub.1. Therefore, the effective injection efficiency is decreased to increase the holding current I.sub.H as compared with that of the structure having no exposed base regions S.sub.1.
The effect of increasing the switching current I.sub.S and the holding current I.sub.H caused by the above operational mechanism is enhanced as the effective lateral resistance R.sub.N of the base region N.sub.1 along the path of the current component I.sub.1 in FIG. 10B is reduced. Therefore, it is apparent that the effect described above is increased as the number of exposed base regions S.sub.1 is increased.
On the other hand, the current component I.sub.0 directly flowing from the base region N.sub.1 to the base region N.sub.2 shown in FIG. 10B is a reactive current that does not contribute to the turn-on operation, which increases only the switching current I.sub.S as an additional current. Although the current component I.sub.2 is also a reactive current, its description will be omitted since it is the same as that in FIG. 8A.
As a result, in the structure shown in FIGS. 10A and 10B, when the number of exposed base regions S.sub.1 is increased, the holding current I.sub.H is sufficiently increased. However, the unwanted current component I.sub.0 flows through the device in proportion to the number of the exposed base regions S.sub.1 and their total area to increase the switching current I.sub.S, thereby degrading the surge operation performance.
In addition, when the thyristor type bidirectional surge suppressor is turned on, a portion of the device to be most easily turned on is first turned on, and the turned-on region extends over the entire area of the device. Therefore, when an extending speed of the turned-on area is slower than the increasing speed of the surge current, a current density is excessively increased in the turn-on process. When the current density exceeds a predetermined limit, the device is broken. For this reason, the surge suppressor cannot sufficiently protect the electronic circuit from the surge current having a short rise time.
Therefore, it is necessary that even when the surge current having the short rise time is applied to the device, the turn-on area is smoothly increased in accordance with the increase in the surge current. However, when the number of exposed base regions S.sub.1 shown in FIGS. 10A and 10B is increased to increase the holding current I.sub.H, the distance among the base regions S.sub.1 is decreased, thereby restricting the extension of the turn-on state over the entire area. For this reason, the surge current capacity is decreased to reduce the surge operation performance.